1. Field of the Invention
The present invention relates to an apparatus and method for adjusting the image contrast of a display device, and more particularly, to an apparatus and method for adjusting the image contrast of the display device which is incorporated in the console panel unit of a copying machine or a facsimile device.
2. Description of the Related Art
Various apparatuses have a console panel unit. Among these apparatuses are: an image processor for processing image information, e.g., characters and graphics; an image scanner for reading image information from an original and supplying this information to an image processor; a printer for printing image data output from a computer or the like, on a recording medium; a copying machine for copying an original having image information, and a facsimile device for transmitting and receiving image information though a communication line. A display device is incorporated in the console panel unit of each of these apparatuses, for displaying messages showing the conditions of some of the components of the apparatus and the instructions as to operate the apparatus.
It is desirable that the display device incorporated in the console panel unit of, for example, a copying machine which performs an electrophotography process, should display as many characters, symbols, and graphic characters as is possible. Further, it is demanded that the display device should display as many messages as is possible, since the apparatus has many functions to perform and is required to carry out these functions in complex combinations. To display many characters, symbols, graphic characters, and messages, the display device used in the copying machine or the like is large in size.
Display devices of various types are known which can be used for the purpose described above. Among these are: a light-emitting diode display (hereinafter referred to as "LED display" which can display characters, graphics, and graphic characters; a cathoderay tube display (hereinafter referred to as "CRT display") which can display characters, graphics, and graphic characters, and a liquid crystal display (hereinafter referred to as "LCD" which ca display the same data items as the CRT display. The LED display cannot display as many characters and graphics as are desired. On the other hand, although the CRT display may display as many characters and graphics as is desired, it is considerably larger. The LCD is not as large as the CRT display but can display as many characters, graphics, and graphic characters as is desired, and is thus used commonly in console panel units. In particular, dot-matrix type LCDs, which have display elements arranged in rows and columns, are used in great numbers.
The LCDs available at present are classified into three types. The first is a positive type wherein some of the display elements become opaque when driven, thus defining characters, symbols, and graphic characters. The second is a negative type wherein some of the display elements become opaque when driven, whereby the remaining display elements define characters, symbols, and graphic characters. The third is a positive-negative type, which functions as a positive type when operated in the first operating mode and as a negative type when operated in the second mode. With most LCDs available now, it is possible to change the image contrast, that is, the ratio in brightness of the data items displayed to the background of the screen.
The image contrast of the LCD is adjusted before the copying machine is delivered from the factory. To be more specific, the drive voltage of the LCD, i.e., the voltage applied from a LCD drive circuit to the LCD is adjusted such that it has a value for which LCDs display images in the best contrast. As is generally known in the art, however, the image contrast of any LCD greatly varies in accordance with the direction in which light is externally applied to the LCD. Hence, the image contrast of the LCD must be readjusted in accordance with the intensity and direction of external light applied to the LCD after the copying machine has been installed in a specific room for use.
The variable resistor used for adjusting the drive voltage of the LCD is incorporated within the console panel unit, and cannot therefore be accessed from outside. In order to have access to the variable resistor, the cover concealing the variable resistor must be removed from the copying machine. After the variable resistor is adjusted, the cover must be attached to the copying machine. Removing and attaching the cover is time-consuming, and increases the chance of staining and damaging the cover. Furthermore, since the variable resistor must be adjusted, with the cover removed, while the power switch of the copying machine is on, the person who adjusts the variable resistor is exposed to the danger of electric shock.